fix(docker): openvpn 2.4.12-r0 install

Dockerfile

@@ -181,7 +181,7 @@ EXPOSE 8000/tcp 8888/tcp 8388/tcp 8388/udp
 HEALTHCHECK --interval=5s --timeout=5s --start-period=10s --retries=1 CMD /gluetun-entrypoint healthcheck
 ARG TARGETPLATFORM
 RUN apk add --no-cache --update -l apk-tools && \
-    apk add --no-cache --update -X "https://dl-cdn.alpinelinux.org/alpine/v3.12/main" openvpn==2.4.11-r0 && \
+    apk add --no-cache --update -X "https://dl-cdn.alpinelinux.org/alpine/v3.12/main" openvpn==2.4.12-r0 && \
     mv /usr/sbin/openvpn /usr/sbin/openvpn2.4 && \
     apk del openvpn && \
     apk add --no-cache --update openvpn ca-certificates iptables ip6tables unbound tzdata && \

internal/configuration/sources/env/openvpn.go

@@ -65,6 +65,11 @@ func (r *Reader) readOpenVPN() (
 		return openVPN, fmt.Errorf("environment variable OPENVPN_VERBOSITY: %w", err)
 	}
 
+	flagsStr := os.Getenv("OPENVPN_FLAGS")
+	if flagsStr != "" {
+		openVPN.Flags = strings.Fields(flagsStr)
+	}
+
 	return openVPN, nil
 }